A small program-type electronic calculator is arranged for a plurality of programs to be stored in a memory. A program number or program bell is attached to the starting section of the respective programs to distinguish the memory regions in which they are stored. When data is going to be written in a memory, the selection of a program number is generally left to the operator's discretion or judgement. Where, therefore, the operator does not remember a program number which is to be specified for the writing of data, it is necessary for him to recognize the used program numbers by checking previously stored data. This operation consumes a great deal of time and work. Further difficulties are that if new data should happen to be stored in a memory region bearing a used program number without carrying out the above-mentioned previous check, then the preceding data stored in a memory region corresponding to a wrongly selected program number would be cleared, leading to the erroneous behavior of the calculator as a whole.
This invention has been accomplished in view of the above-mentioned circumstances, and is intended to provide an unused program number indicating system, which, where a small electronic calculator is set for the write mode, indicates unused program numbers, thereby enabling the operator to be easily informed of the used program numbers.